1. Field of the Invention
The present invention relates to improved methods of cleaning up well bore producing intervals drilled with synthetic oil-water emulsion drill-in fluids.
2. Description of the Prior Art
Synthetic oil-water emulsion drill-in fluids are often utilized to drill the producing interval or intervals in oil and gas wells. That is, a conventional oil or water based drilling fluid is used to drill a well bore to a point just above the producing interval, i.e., the target hydrocarbon containing formation or zone. At that point, the conventional drilling fluid containing drill solids is circulated out of the well bore and replaced with a freshly formulated drill-in fluid. The drill-in fluid is then used to drill the producing interval in an effort to minimize damage to the interval which can limit its productivity.
The fluid properties of the drill-in fluid are carefully controlled so that fluid loss into the producing formation or zone is low. The viscosity of the drill-in fluid is also maintained at a relatively high level to prevent solids in deviated wells from settling to the low side of the well bore whereby displacement of the drill-in fluid is more difficult. The drill-in fluid is a highly stable synthetic oil-water emulsion with the synthetic oil being the continuous phase. In order to add stability to the drill-in fluid, the internal water phase is generally formed of salt water. If the drill-in fluid is weighted, it includes acid soluble weighting materials such as calcium carbonate.
During the drilling of the producing interval with a drill-in fluid of the type described above, filter cake comprised of drill solids and weighting materials builds up on the walls of the well bore. After completion of the well bore through the producing interval, the filter cake along with layers of the drill-in fluid remain on the walls of the well bore thereby plugging the pores and terminating the permeability of the producing interval.
When completion operations are conducted in the producing interval before the drill-in fluid is circulated out, such as the setting of a prepacked screen or other screening device, the filter cake and layers of drill-in fluid on the walls of the well bore are left between the well bore and the screen. When the well is brought on production, the filter cake and drill-in fluid plug at least portions of the screen whereby hydrocarbon production is restricted.
While the filter cake and layers of drill-in fluid remaining in a well bore producing interval have heretofore been cleaned up using acidizing techniques and fluids, a number of problems have been encountered which limit the degree of filter cake and drill-in fluid removal obtained. For example, emulsion blockage on the walls of the well bore and in the formation can occur which limits the contact of an acidizing fluid with the filter cake and drill-in fluid to be removed. The presence of strong oil wetting emulsifiers in the drill-in fluid often prevents or slows the dissolution of acid soluble solids in the filter cake by the acidizing fluid. Also, when the acidizing fluid is displaced over a long producing interval, the rapid dissolution of a portion of the filter cake before the acidizing fluid is displaced through the entire interval can cause the acidizing fluid to be lost into the formation whereby only a portion of the interval is cleaned.
Thus, there are needs for improved methods of cleaning up the producing interval of a well bore drilled using a synthetic oil-water emulsion drill-in fluid whereby the filter cake and drill-in fluid on the walls of the producing interval are completely removed and a high percentage of the original producing interval permeability is regained.